Seminal-type ribonuclease genes in ruminants, sequence conservation without protein expression?

Bovine seminal ribonuclease (BS-RNase) is an interesting enzyme both for functional and structural reasons. The enzyme is the product of a gene duplication that occurred in an ancestral ruminant. It is possible to demonstrate the presence of seminal-type genes in all other investigated ruminant species, but they are not expressed and show features of pseudogenes. In this paper we report the determination of two pancreatic and one seminal-type ribonuclease gene sequences of swamp-type water buffalo (Bubalus bubalis). The two pancreatic sequences encode proteins with identical amino acid sequences as previously determined for the enzymes isolated from swamp-type and river-type water buffalo, respectively. The seminal-type sequence has no pseudogene features and codes for an enzyme with no unusual features compared with the active bovine enzyme, except for the replacement of one of the cysteines which takes part in the two intersubunit disulfide bridges. However, Western blotting demonstrates the presence of only small amounts of the pancreatic enzymes in water buffalo semen, suggesting that also in this species the seminal-type sequence is not expressed. But it is still possible that the gene is expressed somewhere else in the body or during development. Reconstruction of seminal-type ribonuclease sequences in ancestors of Bovinae and Bovidae indicates no serious abnormalities in the encoded proteins and leads us to the hypothesis that the ruminant seminal-type ribonuclease gene has not come to expression during most of its evolutionary history, but did not exhibit a high evolutionary rate that is generally observed in pseudogenes.